Vacuum pump apparatus



g 29, 1967 .1. c. HELMER VACUUM PUMP APPARATUS Filed Marc h 1965INVENTOR. JOHN C. HELMER United States Patent 3,338,506 VACUUM PUMPAPPARATUS.

John C. Helmet, Menlo Park, Calif, assignor to Varian Associates, PaloAlto, Calif., a corporation of California Filed Mar. 5, 1965, Ser. No.437,537 Claims. (Cl. 23069) ABSTRACT OF THE DISCLOSURE An ion-gettervacuum pump. A titanium-wrapped filament electrode in the pump envelopeis electrically heated causing both thermionic emission of electrons andsublimation of the titanium. A second electrode is used to cause theemitted electrons to travel long paths Within the envelope to increasethe probability of collision with gas molecules and the production ofions of gas. The ions deposit on the walls of the envelope where theyare gettered or buried by condensing titanium.

The present invention relates to vacuum pump apparatus and particularlyto such apparatus employing sublimation and ionization techniques.

Pumping by thermally evaporating or subliming getter materials onto theinternal walls of a housing, suitablyforepumped, has been well known formany years. Gas molecules coming into contact therewith combinechemically or physically with the condensed material and are removedfrom the gaseous state so as to reduce the pressure. Using pumpingapparatus of this type, extremely high pumping speeds have beenachieved, in excess of 3500 liters/see, for the active gases such as 0 NCO, and CO which can be scaled up as the need arises.

Sublimation pumping apparatus suffer from the drawback that they areineffective in pumping the inactive gases, such as the noble gases andmethane gas. For this reason sublimation pumping apparatus arefrequently used in combination with some means which is capable ofpumping the inactive gases, such as a diffusion pump or, where extremelylow pressure in a contaminant free fashion are desired, a device whichemploys an ionizing discharge. Typical examples of apparatus whichemploy both sublimation and ionization techniques (hereinafter referredto as combination vacuum pumps) are described in a note entitledEvapor-Ion Pump, by R. G. Herb, R. H. Davis, A. S. Divatia and D. Saxon,Physical Review, second series, vol. 89, No. 4, p. 897, Feb. 13, 1953and inan article entitled Orbitron Vacuum Pump, by R. A. Douglas, J.Zabritski and R. G. Herb, Review of Scientific Instruments, vol. 36, pp.1-6, January 1965.

These prior art combination vacuum pumps are characterized by theprovision of a sublimation means including a source of getter materialadapted to sublime upon heating, onto the interior walls of the pump,and an ionization means including a source of electrons, which sourcemay also 'serve to heat the getter material by electron bombardment, anda means for elongating the path of the electrons between the source andelectrode serving as a collector to increase the probability of ionizingcollisions.

It is the object of the present invention to provide an improved vacuumpump apparatus employing both sublimation and ionization techniques inwhich the need for a separate source of ionizing electrons iseliminated.

Briefly stated, in accordance with one teaching of the present inventionthere is disclosed a combination vacuum pump in which the need for aseparate electrode which acts as a source of electrons is eliminatedcomprising a dual functioning sublimation means which acts both as asource of electrons and getter material, in combination with a means forelongating the path of the electrons.

One feature of the present invention is the provision in a combinationvacuum pump of a dual functioning sublimation means, which acts both asa source of ionizing electrons and getter material, in combination witha means for elongating the path of electrons.

Another feature of the present invention is the provision of acombination vacuum pump of the above type wherein the means forelongating the path of the electrons comprises an electrode, forexample, a grid electrode.

These and other objects and features of the present invention and afurther understanding may be had by referring to the followingdescription and claims taken in conjunction with the following drawingin which:

FIG. 1 is a diagrammatic view of one embodiment of the presentinvention;

FIG. 2 is a diagrammatic view of another embodiment of the presentinvention;

FIG. 3 is a diagrammatic view of still another embodiment of the presentinvention; and,

FIG. 4 is an enlarged fragmentary view of the sublimation means utilizedin the present invention.

Referring now to FIG. 1, there is disclosed a vacuum pump incorporatingthe novel features of the present invention comprising: an evacuableenvelope 11 adapted to contain gas molecules; an electron emissivesublimation means 12, including a source of getter material 13, adaptedto be heated so as to cause electron emission and sublimation of gettermaterial therefrom; a surface 14 for receiving getter material andcollecting positive ions; and, a means 15 for elongating the pathtraveled by the electrons between emission and collection so as toincrease the probability of ionizing collisions between electrons andgas molecules.

Envelope 11 has a cylindrical side wall 16 with a rotationally symmetricinterior wall forming the surface 14, and apertured top 17 and bottom 18Walls, all made of gas impervious material, for example, stainlesssteel. Bottom wall 18 is apertured for receiving, in vacuum tightmanner, high voltage 19 and high current 20 feedthroughs. The envelope11 is adapted to be connected in vacuum tight manner to a structure tobe evacuated (not shown) through the intermediary of a high conductancepassage 21 and vacuum flange 22 welded thereto. During operation, theenvelope 11 is maintained at a potential equal to or greater than thatof the sublimation means 12 so that electrons emitted therefrom will not:be attracted to surface 14.

Axially positioned within envelope 11 is the electron emissivesublimation means 12 supported on high current feedthrough 20. The means12 is electrically isolated from the envelope 11 and is described withmore particularity in US. application Ser. No. 392,417, filed Aug. 27,1964 and assigned to the same assignee as the present invention. As bestseen in FIG. 4, it includes a core 23, typically tungsten, and anoverlay made up of a plurality of getter material wires 13, typicallytitanium, and a single wire 24 of a refractory material such asmolybdenum, helically twisted tightly about the core 23 so that theoverlay is in direct thermal contact with the core 23 throughout itslength. Core 23 is preferably located on the axis of symmetry of thesurface 14. It is believed that this overlay constitutes the source ofelectrons. During operation, the sublimation means 12 is essentially atground potential. A power supply (not shown) of, the example,

5-6 volts is connected to the sublimation means 12 and current passedtherethrough, typically 40-45 amperes, which is high enough to causesublimation of the getter material. In addition, it has been found thatat sublimation temperatures the sublimation means itself is a copioussource of electrons enough to support an ionizing discharge incombination with the means 15. By way of example, in a typicalembodiment at sublimation temperatures of about 1650 C. a temperaturelimited electron emission of about 170 milliamps. is obtained.

To obtain easy starting and low base pressure one may cool the walls ofthe envelope 11, for example, by the provision of a cooling conduit 25brazed to the wall 16 through which a coolant fluid such as water may becirculated, to dissipate heat from the sublimation means 12.

Surface 14 acts as a collector for getter material sublimed from themeans 12 and for gas atoms or molecules ionized by impact, the ionsbeing impelled by electric fields into contact with the sublimed gettermaterial which removes them by absorption, adsorption, or entrapment.

The means 15 may comprise a grid cage of relatively small surface areaspaced from and coaxially surrounding the sublimation means 12,supported on high voltage feedthroughs 19 and insulated from theenvelope 11. During operation, the grid cage is maintained positive withrespect to sublimation means 12 and envelope 11 high enough to drawsaturation electron emission from the means 12, but low enough thationization probability remains high, typically 5004000 volts. Theelectrons emitted by the sublimation means 12 are caused to oscillateback and forth within envelope 11 greatly lengthening the path traveledby the electrons emitted by the means 12. In their flight back andforth, the electrons will gain kinetic energy and may collide with aneutral gas molecule. If the electron has gained sufiicient energy itwill ionize the neutral molecule and produce a free electron. Positiveions formed between the grid 15 and surface 14 will be driven toward thesurface 14, where they may be absorbed by getter material depositedthereon, as well as covered over by subsequently sublimed titanium. Theembodiment of FIG. 1 is particularly advantageous for ion pumping, sincethe region of maximum ion bombardment coincides with the region ofmaximum deposition of getter material, Some ions created between thegrid 15 and means 12 may be driven back towards the means 12 andtherefore may not be pumped. This loss is minimized by making the radiusof the grid a small fraction of the radius of the chamber, for example,a grid of one inch radius in an envelope with an inside radius of threeinches.

FIG. 2 discloses still another embodiment of the present inventionsimilar to the embodiment of FIG. 1 but modified in that the sublimationmeans 12 and grid cage 15 now enlarged, are disposed in side-by-siderather than coaxial relationship with each other. In this embodiment itis believed that there is a higher probability that ions formed withinthe grid cage will reach the collector surface 14.

FIG. 3 discloses an alternate embodiment of the present invention inwhich a large portion of the electrons emitted by the sublimation means12 are orbited about an axial rod 26 of small surface area in asymmetric electrostatic field between surface 14 and axial rod 26. Thepotential of surface 14 may be equal to or greater than the potential ofthe grounded sublimation means 12 while the axial rod 26 is held at ahigh positive potential by a power supply means (not shown), typically-10kv. The sublimation means 12 is disposed parallel to the axis ofsurface 14, being spaced between the rod 26 and surface 14 so thatadequate electron emission and long electron paths may be achieved. Theelectrons so orbited serve to ionize gas molecules which are driven tothe collector surface 14 where they may be absorbed and subsequentlycovered over by the sublimed getter material. This embodiment has theadvantage that extremely long electron paths between emission andcollection may be obtained if the diameter of the rod 26 is sufiicientlysmall and voltage is sufficiently high. This embodiment is claimed incopending application Ser. No. 437,536, of Wesley H. Hayward, filed Mar.5, 1965, and assigned to the same assignee as the present invention.

Since many changes can be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A vacuum pump apparatus comprising:

(a) an evacuable envelope adapted to contain gas molecules;

(b) electron emissive sublimation means disposed within said envelopeand including a source of getter material, said means adapted to beheated so as to cause electron emission and sublimation of gettermaterial therefrom;

(c) a surface disposed Within said envelope for receiving gettermaterial and collecting positive ions; and

(d) means for elongating the path of said electrons between emission andcollection so as to increase the probability of ionizing collisionsbetween said electrons and said gas molecules, said means for elongatingcomprising a wire mesh cage enclosing a volume of space within saidenvelope.

2. The apparatus according to claim 1 wherein said wire mesh cage isspaced from said sublimation means and said surface and adapted to bemaintained at a potential positive with respect to said surface.

3. The apparatus according to claim 2 including means for cooling saidenvelope in intimate contact therewith.

4. The apparatus according to claim 1 wherein said envelope has arotational symmetric interior wall defining said surface, saidsublimation means lies on the axis of symmetry of said wall and saidwire mesh cage is spaced from said sublimation means and adapted to bemaintained at a potential positive with respect to said surface.

5. The apparatus according to claim 4 wherein said wire mesh cage iscoaxially disposed about said sublimation means.

References Cited UNITED STATES PATENTS 2,131,897 10/1938 Malter 230-692,888,189 5/1959 Herb 23069 2,925,504 2/1960 Cloud et al. 313-72,979,254 4/ 1961 Reich 230-69 2,988,657 6/1961 Klopfer 313-7 3,244,9694/1966 Herb et al. 32433 ROBERT M. WALKER, Primary Examiner,

1. A VACUUM PUMP APPARATUS COMPRISING: (A) AN EVACUABLE ENVELOPE ADAPTEDTO CONTAIN GAS MOLECULES; (B) ELECTRON EMISSIVE SUBLIMATION MEANSDISPOSED WITHIN SAID ENVELOPE AND INCLUDING A SOURCE OF GETTER MATERIAL,SAID MEANS ADAPTED TO BE HEATED SO AS TO CAUSE ELECTRON EMISSION ANDSUBLIMATION OF GETTER MATERIAL THEREFROM; (C) A SURFACE DISPOSED WITHINSAID ENVELOPE FOR RECEIVING GETTER MATERIAL AND COLLECTING POSITIVEIONS; AND (D) MEANS FOR ELONGATING THE PATH OF SAID ELECTRONS BETWEENEMISSION AND COLLECTION SO AS TO INCREASE THE PROBABILITY OF IONIZINGCOLLISIONS BETWEEN SAID ELECTRONS AND SAID GAS MOLECULES, SAID MEANS FORELONGATING COMPRISING A WIRE MESH CAGE ENCLOSING A VOLUME OF SPACEWITHIN SAID ENVELOPE.